A spark plug for use in an internal combustion engine, such as an automobile engine, includes, for example, a center electrode extending in an axial direction, an insulator provided outside the center electrode, a cylindrical metal shell provided outside the insulator, and a ground electrode having a base end extending from a leading end of the metal shell. The ground electrode has a rectangular shape in cross section, and an inner side surface of a distal end of the ground electrode is arranged so as to oppose a leading end of the center electrode, whereby a spark discharge gap is formed between the leading end of the center electrode and the distal end of the ground electrode. A conceivable attempt has been made to weld a tip (a noble metal tip) made of a noble metal alloy to the leading end of the center electrode and/or the distal end of the ground electrode, thereby enhancing spark wear resistance.
In order to enhance power and fuel consumption, and realize maintenance-free performance, there exists a demand for a spark plug exhibiting both ignitability and durability. Accordingly, in an attempt to enhance ignitability, there has been proposed a technique for welding a noble metal tip having a comparatively small diameter to a center electrode and welding the noble metal tip to the distal end of the ground electrode while the volume of the distal end of the ground electrode is being comparatively reduced (see; for instance, Patent Document 1 and the like).    Patent Document 1: JP-B-3980279